1. Technical Field
The present invention relates to an automatic gain controller, and more particularly, to a transmitting apparatus for use with a mobile communication terminal having the automatic gain controller.
2. Discussion of the Related Art
A transmitting apparatus, for example, a transmitter, of a mobile communication terminal, such as a cellular phone, consumes the majority of its battery power when transmitting data input by a user or searching for a base station. For example, when the user hits a calling key of the mobile communication terminal or sends a text message, in other words, when the user desires to transmit data, the transmitting apparatus receives a data stream or symbols related to phone number information or text message information. The transmitting apparatus then converts the received data stream or the received symbols into a packet according to a code division multiple access (CDMA) standard, modulates the packet, and transmits the modulated packet to a base station as a radio frequency (RF) signal over a carrier wave channel.
The mobile communication terminal also consumes a considerable amount of power when searching for a base station in a roaming state. This occurs when the mobile communication terminal is in the process of determining if it is within a communication range of the base station by transmitting/receiving a channel check signal to/from the base station to determine if it can connect to an allotted communication channel. Thereafter, the mobile communication terminal displays a result of the determination in a manner that can be easily recognized by a user possessing the mobile communication terminal by using, for example, a liquid crystal display (LCD) or a light emitted diode (LED).
Because the mobile communication terminal transmits predetermined data input by the user or the channel check signal to the base station wirelessly after amplifying the predetermined data or the channel check signal using an intermediate frequency (IF) amplifier and an RF amplifier, the power consumption of the mobile communication terminal is dependent on the gain control processes carried out by the RF amplifier and the IF amplifier. A method of reducing power consumption by controlling gain using an automatic gain controller has been disclosed in International Patent Application No. PCT/US2000/16269, entitled “Adjusting Maximum Transmit Power to Maintain Constant Margin for Adjacent Channel Power Rejection.
FIGS. 1A and 1B are graphs illustrating the characteristics of amplifiers of a conventional transmitting apparatus of a mobile communication terminal. More specifically, FIG. 1A illustrates the gain characteristics of an IF terminal, and FIG. 1B illustrates gain the characteristics of an RF terminal. In FIG. 1A, (a), (b), and (c) represent variations of power, signal-to-noise ratio (SNR), and spurious noise, respectively at the IF terminal with respect to the variation of a control voltage. Likewise, in FIG. 1B, (d), (e), and (f) represent variations of power, SNR, and spurious noise, respectively at the RF terminal with respect to the variation of the control voltage. As shown in FIGS. 1A and 1B, specific gain measurements are not illustrated. Instead, FIGS. 1A and 1B illustrate gain variation tendencies of power, SNR, and spurious noise, at the IF terminal and the RF terminal, respectively with respect to the control voltage.
According to the CDMA standards, the power of a mobile communication terminal is controlled such that a transmitting apparatus has a gain of +70 dB or higher. In most conventional transmitting apparatuses, such gain characteristics can be obtained by using an automatic gain controller disposed at an IF terminal. In addition, some conventional transmitting apparatuses include a variable gain amplifier, which is disposed at a driver terminal. As shown to the left of the vertical dashed line of FIG. 1A, the spurious noise (c) is maintained at a constant level without regard to the decrease of gain controlled by a predetermined control voltage level, because of a heat noise of approximately −174 dBm/Hz. Accordingly, the SNR (b) gradually decreases in accordance with the decrease of the control voltage as long as the spurious noise (c) is maintained at the constant level.
In FIG. 1B, unlike in FIG. 1A, the spurious noise (f) gradually decreases in accordance with the decrease of the control voltage, and the SNR (e) is maintained at a constant level without regard to the variation of the control voltage. Therefore, the total gain characteristics of the transmitting apparatus are characterized by the SNR being varied when performing a gain control process using the IF terminal, and the SNR is maintained at a constant level when performing a gain control process using the RF terminal.
FIGS. 2A and 2B are graphs illustrating the gain characteristics of a conventional transmitting apparatus depending on how much gain an automatic gain controller distributes to an RF terminal and an IF terminal. In FIGS. 2A and 2B, (a) and (b) represent gain at the RF terminal and gain at the IF terminal, respectively.
In FIG. 2A, a signal that is output from the conventional transmitting apparatus has the same variable gain characteristics as shown in FIG. 2B. In the conventional transmitting apparatus, a desired variable gain range and a desired output power level can be obtained by adjusting the gain (a) at the RF terminal and the gain (b) at the IF terminal. According to the CDMA standards, adjacent channel power rejection (ACPR) characteristics, which are defined therein, are measured at a maximum output power of +23 dBm or higher, and when the gain at the RF terminal of the conventional transmitting apparatus is higher, its ACPR characteristics are enhanced. On the other hand, the spurious emission characteristics, which are defined in the CDMA standards, are measured at a maximum output power of −13 dBm, and when the gain at the IF terminal increases, it is more likely to obtain a higher SNR having a spurious tone and a noise floor.
When the gain (a) at the RF terminal is larger than the gain (b) at the IF terminal, as shown in FIG. 2A, noise may be considerably amplified. Thus, it may be difficult to obtain enhanced spurious emission characteristics. On the other hand, when the gain (b) at the IF terminal is larger than the gain (a) at the RF terminal, as shown in FIG. 2B, a high SNR can be obtained from the IF terminal. Thus, it is not difficult to obtain desired spurious emission characteristics. However, because a signal with a high magnitude is typically applied to the RF terminal, it is relatively difficult to obtain linearity and enhanced ACPR characteristics.
The maximum power of a typical transmitting apparatus of a mobile communication terminal generally ranges from +23 dBm to +25 dBm, and the typical transmitting apparatus consumes a current of about 600-700 mA when gain at each amplifier approximately reaches its maximum. The current consumption of the typical transmitting apparatus is considerably affected by a slight variation of ±1 dB in the power of the typical transmitting apparatus. Therefore, unless a maximum gain at each amplifier is precisely controlled in a mobile communication terminal using a battery as a power source, the mobile communication terminal inefficiently uses or dissipates current and the battery's life is shortened.
Accordingly, there is a need to control the gain of an RF amplifier and an IF amplifier in a mobile communication device in order for the device to transmit, for example, a CDMA signal without consuming a considerable amount of power